The present invention relates generally to gas turbine engines, and, more specifically, to inlet guide vanes therein.
A turbofan gas turbine engine configured for powering an aircraft in flight includes in serial flow communication a fan, low and high pressure compressors, combustor, and high and low pressure turbines. Air is pressurized in the compressors and mixed with fuel in the combustor for generating hot combustion gases which flow downstream through the turbines which extract energy therefrom. The high pressure turbine powers the high pressure compressor through a shaft therebetween, and the low pressure turbine powers the fan and low pressure compressor through another shaft therebetween.
The engine operates at various power levels including idle, cruise, takeoff, and maximum power as required for the proper operation of the aircraft over its intended flight envelope. In a typical commercial passenger aircraft application, fuel consumption is a primary design objective, and the engine is therefore specifically designed to maximize fuel efficiency at cruise operation.
Accordingly, the various stator airfoils and rotor blades in the fan, compressor, and turbines are preferably configured for maximizing aerodynamic performance at the cruise design point or condition. Correspondingly, aerodynamic performance changes at the off-design conditions not associated with cruise.
Such off-design operation is particularly significant in variable inlet guide vanes in the compressor. The typical turbofan compressor is a multistage axial compressor having many rows of stator vanes and rotor blades through which air is pressurized in turn. Fixed stator vanes are typically used in the downstream stages of the compressor, with variable stator vanes being used in the upstream stages thereof.
And, a row of variable inlet guide vanes is provided at the entrance of the high pressure compressor for optimizing performance thereof over the desired flight envelope including cruise to maximum power operation of the engine. The various compressor vanes and blades are therefore configured in aerodynamic profile for maximizing compression efficiency without unacceptable flow separation or undesirable compressor stall.
Efficient aerodynamic profiles for compressor airfoils including stator vanes and rotor blades have been available for many decades, and may be found in considerable detail in reports prepared by the National Advisory Committee for Aeronautics (NACA). For example, in NACA Technical Note 3959, published in May 1957, vane profiles and design charts are presented for the NACA 63-006 series of 6-percent-thick guide vanes. The specific airfoil series presented in this technical note is the NACA 63-(Cl0A4K6)06 guide-vane profile.
This NACA-63 series vane profile includes a maximum thickness-to-chord length of six percent (6%), which is located at thirty-five percent (35%) chord length from the leading edge of the airfoil. The corresponding leading edge radius is 0.297 percent chord, and the trailing edge radius is 0.6 percent chord. This profile is readily scalable to include maximum thicknesses greater than 6% C, including 8% C and higher, at the same 35% chord location.
The NACA 6% guide vane profile is one of a series of profiles which vary in configuration including maximum airfoil thickness. The various NACA profiles have been available for decades for use in designing efficient gas turbine engine compressor airfoils.
For example, the NACA-63 series airfoil is found in turbofan gas turbine engines enjoying many years of successful commercial use in this country, as well as abroad. In particular, the NACA-63 series profile has been used in a variable inlet guide vane of a high power commercial turbofan aircraft engine.
The NACA profile has good aerodynamic performance and efficiency at the specific design condition for the variable vane, which is typically at the cruise angle of attack. Accordingly, when the guide vane is rotated to its nominal or zero angular position corresponding with the design condition, such as cruise, the aerodynamic profile thereof provides acceptable performance without undesirable flow separation of the air flowing thereover.
However, the variable vane must be rotated over a range of turning angles having correspondingly different angles of attack relative to the airflow entering the compressor. This angular range includes a relatively closed angular position associated with low power or idle operation of the engine at one extreme, and at the opposite extreme of angular position the vanes are rotated to a relatively open position corresponding with maximum power operation of the engine. And, the vanes are positioned between these two opposite end positions for intermediate power operation of the engine such as cruise over a corresponding cruise range of turning angles centered at zero degrees.
In a current development program for a derivative turbofan engine, a wide range of variable inlet guide vane angular position is desired including twenty-four degrees (24xc2x0) maximum open position and sixty degrees (60xc2x0) maximum closed position for a total range of eighty-four degrees (84xc2x0). This wide range of vane angular position has correspondingly wide and different angles of attack relative to the incoming airflow.
Analysis has uncovered substantial flow separation on the pressure side of the vane at the maximum open angular position, and along the suction side of the vane at the maximum closed position for the conventional NACA-63 series of guide vane profiles. Although good compressor performance of the variable inlet guide vanes may be obtained at and near the design condition corresponding with a narrow angular range centered on zero degrees, the compressor will experience poor off-design performance including undesirable flow separation at the opposite ends of the wide angular range of operation.
Accordingly, it is desired to provide an improved variable inlet guide vane having improved off-design performance over a wide angular range of operation.
A variable inlet guide vane includes opposite pressure and suction sides extending along a chord between leading and trailing edges and in span from root to tip. The vane has a maximum thickness greater than about eight percent chord length and located less than about thirty-five percent chord length from the leading edge.